Status of C99 features in GCC

C99 is substantially completely supported as of GCC 4.5
(with -std=c99 -pedantic-errors used;
-fextended-identifiers also needed to enable extended
identifiers before GCC 5), modulo bugs and floating-point
issues (mainly but not entirely relating to optional C99 features from
Annexes F and G). The following table gives more details of the C99
support in different GCC versions.

This table is based on the list in the foreword to N1256
(ISO/IEC 9899:1999 (E), consolidated with ISO/IEC 9899:1999/Cor.1:2001
(E), ISO/IEC 9899:1999/Cor.2:2004 (E) and ISO/IEC 9899:1999/Cor.3:2007
(E)).

The "Version" column indicates the first GCC version in which
support for the relevant feature was substantially present; some bugs
or corner cases may have been fixed in later versions; this column is
"N/A" if nothing is needed from the compiler for the feature to be
substantially supported (for example, if the feature refers to
addition of new library functions rather than language features), even
if additional compiler features could be useful in conjunction with
it. It is assumed that GCC is used with -std=c99
-pedantic-errors (for versions 3.0 and later), as well
as -fextended-identifiers in the case of that feature
before GCC 5.
Where library cooperation is required, it is assumed that a recent
version of the GNU C Library is in use, and support with other C
libraries may be less good. Where the version listed is before GCC
3.0, it should not be assumed that all corner cases follow C99 before
GCC 3.0, even if there is no specific note regarding corner cases.

See below the table for further notes on some issues.

Feature

Version

Notes

restricted character set support via digraphs and
<iso646.h> (originally specified in AMD1)

GCC 2.7

wide character library support in
<wchar.h> and <wctype.h>
(originally specified in AMD1)

Optimization, no compiler support required. GCC has
optimized based on aliasing rules since GCC 2.95.

restricted pointers

GCC 2.95

variable length arrays

GCC 0.9

Various corner cases fixed in GCC 4.5.

flexible array members

GCC 3.0

static and type qualifiers in parameter array declarators

GCC 3.1

complex (and imaginary) support in <complex.h>

GCC 3.0

New functions are a library issue not requiring much compiler
support (some built-in functions present). Complex numbers are
supported with __complex__ since GCC 2.5, and with
C99 _Complex since GCC 3.0. Complex multiplication
and division support C99 special cases since GCC 4.0. Various
corner cases were fixed in GCC 4.5. GCC does not support the
Annex G imaginary types, but this support is optional, and complex
multiplication and division have excess overflows at runtime
(although not beyond those permitted by C99).

type-generic math macros in <tgmath.h>

N/A

Library feature; GCC built-in functions may be used in
implementing it.

the long long int type and library functions

≤ GCC 1.27

New functions are a library issue not requiring much compiler
support (some built-in functions present).

increased minimum translation limits

GCC 0.9

GNU policy has always avoided arbitrary limits.

additional floating-point characteristics in <float.h>

GCC 3.0

remove implicit int

GCC 3.0

reliable integer division

GCC 0.9

universal character names (\u and \U)

GCC 3.1

extended identifiers

GCC 4.1

Some corner cases were fixed in GCC 5;
-fextended-identifiers was needed to enable this
feature before that version.

Conversion specifiers are a library issue (format checking
support present).

compound literals

GCC 3.1

The syntax was supported by GCC by version 1.21, but with
significant differences from C99 requirements until GCC 3.1.

designated initializers

GCC 3.0

The syntax was supported since GCC 2.5, but with significant
differences from C99 requirements until GCC 3.0.

// comments

GCC 2.7

extended integer types and library functions
in <inttypes.h>
and <stdint.h>

N/A

All of this can be provided by the library rather than the
compiler (some built-in function support also
present). <stdint.h> is provided by GCC (as of
version 4.5), or fixed where the system headers provide a
nonconforming version, on some but not yet all systems. On
systems where types in this header have been defined
as char, GCC retains this definition although it is
not permitted by C99.

remove implicit function declaration

GCC 3.0

preprocessor arithmetic done in intmax_t/uintmax_t

GCC 3.3

mixed declarations and code

GCC 3.0

new block scopes for selection and iteration statements

GCC 3.0

integer constant type rules

GCC 3.3

integer promotion rules

GCC 4.0

macros with a variable number of arguments

GCC 2.95

the vscanf family of functions
in <stdio.h> and <wchar.h>

N/A

Library feature, no compiler support required (format checking
support present).

Library feature, no compiler support required (format checking
support present).

inline functions

GCC 4.3

Inline function support present since at least GCC 1.21, but
with major differences from C99 semantics until 4.3.

the snprintf family of functions in <stdio.h>

N/A

Library feature, no compiler support required (format checking
support present).

boolean type in <stdbool.h>

GCC 3.0

GCC 2.95 had <stdbool.h>, but based on an
early draft with major differences from C99 semantics.

idempotent type qualifiers

GCC 3.0

Always has been allowed, with a warning as required by C90
depending on GCC version.

empty macro arguments

GCC 0.9

Undefined behavior in C90, but GCC not known ever to have
handled them contrary to C99.

new structure type compatibility rules (tag compatibility)

GCC 0.9

This relates to compatibility between translation units.

additional predefined macro names

GCC 3.0

Support for the compiler to implicitly preinclude a
file stdc-predef.h provided by the C library, and so
predefine macros relating to library properties for the whole
translation unit, is new in GCC 4.8.

_Pragma preprocessing operator

GCC 3.0

standard pragmas

Not implemented. Associated command-line options to control
the state of the pragmas for the whole translation unit are
available.

__func__ predefined identifier

GCC 2.95

va_copy macro

GCC 3.0

additional strftime conversion specifiers

N/A

Library feature, no compiler support required (format checking
support present).

LIA compatibility annex

N/A

This annex describes how C relates to another standard, and
does not impose any requirements on C implementations.

deprecate ungetc at the beginning of a binary file

N/A

Library feature, no compiler support required.

remove deprecation of aliased array parameters

GCC 0.9

GCC has never done anything regarding this deprecation.

conversion of array to pointer not limited to lvalues

GCC 3.1

Some support since GCC 2.0, but with major differences from
C99 requirements before GCC 3.1.

relaxed constraints on aggregate and union initialization

≤ GCC 1.21

relaxed restrictions on portable header names

GCC 0.9

GCC has never had such restrictions itself.

return without expression not permitted
in function that returns a value (and vice versa)

GCC 3.0

Further notes

cpp has limited support for multibyte character sets.

IEC 60559 is IEEE 754 floating point. This works if and only if
the hardware is perfectly compliant, but GCC does not define
__STDC_IEC_559__ or implement the associated standard
pragmas; nor do some options such as -frounding-math to
enable the pragmas globally work in all cases (for example, required
exceptions may not be generated) and contracting expressions (e.g.,
using fused multiply-add) is not restricted to source-language
expressions as required by C99.

Compiler support is needed for thorough support of math_errhandling; see
messages 1,
2,
3
on this subject to libc-hacker. The compiler needs to mark its output
from compilations using -fno-trapping-math
or -fno-math-errno, possibly using
the .gnu_attribute mechanism, to indicate that built-in
function optimizations may have been applied that mean that not all
calls report error status in a particular way; the static linker
needs to put this information in executables and shared libraries and
the C library needs to use it to set math_errhandling at
startup to a conservatively correct value based on the information
from the compiler. There is currently some limited GNU C Library
support that only conforms as long as the above options are not used
anywhere in the program.

const-qualified compound literals could share storage
with each other and with string literals, but currently don't.

The information provided by static in parameter array
declarators is not used for optimization. It might make sense to use
it in future in conjunction with work
on prefetching.

For questions related to the use of GCC,
please consult these web pages and the
GCC manuals. If that fails,
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